BALLOON (Fr. &Ilion, a large ball). According to the principle of Archimedes (q.v.), bodies immersed in a fluid are buoyed upwards with a force equivalent to the weight of the fluid displaced by them. If their own weight is not sufficient to counterbalance this force—that is. if they are lighter than the fluid—they rise upwards with a force equal to the difference of the weight of the displaced fluid and of their own weight. A B., therefore, which consists of an integument inclosing a gas within it, will rise in air in the wale way that a cork rises in water, provided that the weight of the whole be less than that of an equal volume of air. If one, for instance, occupy as much space as 1000 lbs. of air, but weigh itself—covering, gas, and appendages-600 lbs., it will be impelled upwards with a force of 400 lbs. The gases employed for filling balloons are either hydrogen or ordinary coal-gas. The, former, when pure, is between 14 and 15 times lighter than atmospheric air, and the latter generally about two and h half.
The B., as it is at present employed, is a large pear-shaped bag, in of any pliable silk cloth, covered with a varnish. made by dissolving caoutchouc in oil turpentine, to render it air-tight. The common size of this bag varies from 20 to 30 ft. in equatorial diameter with a•proportionate height. The mouth or neck of this bag just large enough to enable a man to get inside to make any necessary repairs, and is, of course, turned turned downwards when the B. is inflated. A net-work i of hempen or cotton accurately fitted to the B., and the separate cords, on which it ends, are tied to a circular hoop placed a few feet below the neck. The car, generally a large wicker-basket, is sus pended by ropes from this hoop, and hangs at a considerable distance below, so that the aeronaut may be removed from the vicinity of the gas. The net-work serves to dis tribute the weight of the car and its charge equally over the whole upper surface of the i balloon. One of the most important requisites in the construction is the valve, which is introduced into the top of the balloon. It consists of a wooden clapper, 4 or 5 in. square, opening inwards, and kept closed by a sufficient spring. A rope attached to this valve descends through the neck into the car, where, to prevent accidental opening, it is allowed to dangle freely. The furniture of the car are the ballast or sand-bags, by emptying which the B. maybe lightened; the barometer, or corresponding apparatus for telling the height ascended, or the upward or downward course of the B. ; the map and compass, for showing the direction of the voyage; and the grappling-iron, tied to the end of a long rope, for anchoring the B. at the descent. During his flight, the aeronaut has at his disposal the means of guiding his airship only in an upward or downward direction, the motion of translation being wholly dependent on the wind by which it is borne. If he wishes to ascend, he throws some of the ballast over the side of the car; and if to descend, he pulls the valve-rope, so that, the gas rushing by virtue of its specific light ness through the passage made for it by the open valve, the buoyant material may be lessened. It is evident that the power of thus directing his machine becomes more limited with each exercise of it, for in each case there is an unrepaired loss of the means necessary to it. All attempts at guiding balloons in a horizontal direction have hitherto proved failures. In ordinary flights, the mouth of the B. is left open,
so that there is no danger of explosion arising from the expansion of the gas in the rarer regions of the atmosphere. The diffusion that takes place through the open neck is inconsiderable during the few hours that an aerial voyage lasts. Early aeronauts, who kept their balloons closed, frequently ran considerable risk by inattention to the valve when the imprisoned gas demanded vent for its expansion.
The art of traversing the air by means of balloons, generally called aeronautics, and sometimes aerostation, is of comparatively recent date. The germ of the invention of balloons is to be found in the discovery by Cavendish, in 1766, of the remarkable light ness of hydrogen gas, then called inflammable air. Prof. Black, of Edinburgh, seems to have been the first who conceived the idea that a light envelope, containing this gas, would rise of itself, lie requested Dr. Monro, the professor of anatomy, to give him sonic thin animal membrane for the experiment, but for some reason or other, it was never made. The first practical attempts were made by Cavallo, who, in 1772, tilled swine's bladders and paper-bags with the gas, but found the former too heavy, and the latter too porous; and he only succeeded m raising soap-bubbles inflated with the gas. The invention of the B. is due to the two brothers Stephen and -Joseph Montgoltier, paper-makers at Annonay, in France, whose names are as distInguished in the develop ment of their own branch of manufacture as in the history of aeronautics. It immedi ately 'Amick these brothers, on reading Cavendish's Different Kinds of Air, that the air could be rendered navigable by inclosing a light gas within a covering of inconsiderable weight. Led by their avocation, they fixed upon paper as the most fitting material for the purpose, and first attempted to make balloons of paper filled with inflammable air. Finding that these emptied themselves almost as soon as they were filled, instead of abandoning the paper as an unsuitable covering. for the gas, they sought after another as more suited to the paper. By a chain of false reasoning which need not here be detailed, they tIwnglit they found such in the as which resulted from the combustion of slightly moistened straw and which had, as they imagined, an upward tendency, not only from its being heated, but from its electrical properties, which caused it to be repelled from the ground. It is hardly necessary to say that this so-called Montgolfier gas possessed no advantages for raising balloons other titan that possessed by heated air of kind; in fact, the abundant smoke with which it was mixed, by adding to its weight, rather detracted from its merits. At Avignon, in Nov., 1782, Stephen Montgol fler first succeeded in causing a silk parallelopiped, of about 50 cubic ft., to rise to the roof of a room. Encouraged by this success, the brothers made experiments on a larger scale at Annonay with an equally happy result; and finally, in June, 1783, in the pres ence of the states of Vivarams, and of an immense multitude, they raised a B., 35 ft. in diameter, to a height of 1500 feet. This last, nearly spherical in shape, was made of paelteloth, covered with paper, and was heated by an iron choffer placed beneath it, in 1 which 10 lbs. of moist straw and wool were burned.